Two-component polyurethane coatings are long-established. For high-quality lightfast topcoat materials, coatings employed are, in particular, those formed from aliphatic polyisocyanates and polyols such as, for example, polyacrylate polyols, polyester polyols or polycarbonate polyols. Polyisocyanates employed for these high-quality coatings are, in particular, low-monomer-content derivatives prepared from hexamethylene 1,6-diisocyanate (HDI). Particularly suitable for elastic, highly robust coatings are isocyanurate polyisocyanates of HDI, or HDI polyisocyanates containing iminooxadiazinedione and isocyanurate structures, of the kind described for example in H. J. Laas, R. Halpaap, J. Pedain, J. prakt. Chem. 1994, 336, 185-200 or in EP 0330966 B1 or EP 0798299 B1. These polyisocyanates generally have isocyanate functionalities (FNCO) of 3 or more, where isocyanate functionality is understood as the average number of NCO groups per molecule.
Besides HDI, 1,5-diisocyanatopentane (PDI) is another long-established monomer, as described for example by W. Siefken, Liebigs Ann. Chem. 1949, 562, page 122 or in DE 1493360 B1 and DE 1900514 A1.
The greatest part of the areas of application, in volume terms, are covered by solventborne one-component and two-component polyurethane coating materials. Within the broad spectrum of industrial application, these materials are applied usually by suitable spraying methods. Serving as a reference point in the respective application, as a general rule, is the solids content at spray viscosity. Too low a solids content is manifested, for example, in a poorer hiding power and/or lower filling force or reduced productivity.
In order to go along with the trend of increasing requirements with regard to environmental protection, the coatings industry has increasingly undertaken efforts to provide particularly low-viscosity polyisocyanates. A disadvantage of such low-molecular systems is that at the preparation stage, the polymer reaction has to be terminated while still at a very low degree of polymerization of the monomer, in order to achieve the required viscosity.
It is indeed possible for the low-molecular polyisocyanate to be separated by suitable methods from the excess monomer, and for the monomer retained to be replaced by the amount of that polymerized with new monomer, and reacted again. Overall, however, this has a significantly adverse influence on the space-time yield of such a preparation process.
DE 19611849 discloses reducing the viscosity of the polyisocyanate thus prepared through the deliberate simultaneous synthesis of symmetrical and asymmetrical polyisocyanurate structures, although the solids fractions realizable by this means in formulated coating systems are no more than marginally higher. The efforts in stated texts at reducing viscosity have therefore not achieved the desired effect of significant reduction in the solvent fraction in coating formulations. Moreover, the disadvantageous space-time yield has remained unresolved.
Accordingly, it continued to be desirable to be able to realize the aforementioned positive properties of a high space-time yield with a maximum amount of reactive isocyanate groups and with the solids contents at spray viscosity that are required by the coatings industry, in coating formulations comprising a polyisocyanate.